The present invention relates to pressurized liquid storage tanks in general and more particularly to liquid storage tanks which use a biased plunger or means other than air pressure to maintain the liquid under the desired pressure.
Water storage tanks have, of course, been around for eons, including for example the simple gravity type, wherein rain water collected from the eaves of a roof is stored in a tank or cistern in the attic and then provided to the user at a lower level. However, such a technique is not suitable for water obtained from a well unless the water is first pumped to a level higher than the using facility. Since a huge or tall water tower is not always desirable, other techniques of storing water so that it would be available for use throughout a building, were found to be necessary. To this end water is often stored under pressure. To obtain the desired pressure, early water tanks were simply air tight tanks which included enclosed air under pressure as well as water. This pressurized tank is connected directly to the water system. Therefore, according to this system, the tank is first pressurized with air so that as water is forced by the well pump into the tank, the air would be further compressed. The pump continues to force water into the tank displacing the air, until the compressed air in the tank itself exerts as great a force as is provided by the pump. When water is withdrawn from the tank the water leaves the tank under pressure until the water level is reduced to a level, where the air pressure against the water within the tank is reduced to a level sufficient to activate a pressure switch which in turn activates the water pump. Such pressure tanks have significant disadvantages in that the compressed air within the tank will be absorbed by the water. This results in a condition where very little air can be withdrawn from the tank before the pressure is dropped to a point whereby the pump must again be activated. Thus, after a period of time and because the air is absorbed by the liquid, little air remains to be compressed, which causes continuous cycling of the pump. This in turn results in heavy wear and eventual malfunction of the pump. In addition, even under the best of conditions the actual amount of water that can be withdrawn from this type pressure tank constitutes a very small percentage of the actual tank volume. For example, a 42 gallon capacity tank properly charged with air will only hold approximately 20 gallons of water which in turn can be drawn down only about 6 gallons before the pump will reactivate.
To overcome some of the deficiencies of this type air pressure tank, improvements which use a bladder or diaphram within the tank to separate the air from the liquid are available. For example, by providing two compartments, one for air and one for water, which compartments are separated by a flexible diaphragm the mixing of air and water can be avoided. For example, see U.S. Pat. No. 2,594,833 issued to S. M. White on Apr. 29, 1952 and U.S. Pat. No. 3,346,014 issued to C. Jacuzzi on Oct. 10, 1967. Each of these patents, show the two compartment tank using a flexible diaphragm between the two tanks to prevent the mixing of air and water. The White patent further includes a perforated inflexible metal separator to prevent over stressing of the flexible diaphram to prevent early failure. In a similar manner, the Jacuzzi patent includes a bellows type diaphram so that it is not stretched. However, even this type of diaphram still requires flexing during operation. Unlike the present invention both of these two patents rely completely on the use of air pressure for operation. However, U.S. Pat. No. 3,487,855 issued to J. W. Lautenberger on Jan. 6, 1970 discloses a pulsation dampner for controlling the pulsating or water hammer effect of a liquid system under pressure by means of a piston device which receives pressure from the system on both sides of the piston. However, according to this patent, one side is provided with smaller tubing.